Conventional hidden hinge MEMS mirrors, such as those disclosed in U.S. Pat. No. 5,212,582 issued May 18, 1993 in the name of William Nelson, and U.S. Pat. No. 6,535,319 issued Mar. 18, 2003 in the name of Victor Buzzetta et al, include a mirror mounted on the end of a pedestal, extending from a substrate, which are rotatable about a single axis and actuated by electrodes patterned on the substrate below each side of the mirror. In an effort to provide biaxial rotation, Nasiri et al, disclose a MEMS mirror with a complicated hidden lever system, in U.S. Pat. No. 6,533,947 issued Mar. 18, 2003. Unfortunately, the device disclosed in Nasiri et al requires four independent levers and four sets of electrodes equally spaced 90° from each other beneath the mirror, thereby requiring a mirror with a large surface area. Furthermore, an array of such mirrors could not be tightly packed together for reflecting individual wavelengths of light, which has been dispersed in an optical switch. Jung et al disclose a somewhat higher fill factor micro-mirror in an article entitled: “High Fill-Factor Two-Axis Gimbaled Tip-Tilt-Piston Micromirror Array Actuated by Self-Aligned Vertical Electrostatic Combdrives in the Journal of Microelectromechanical Systems, Vol 15, No. 3, pages 563 to 571, June 2006; however, the actuation thereof requires eight sets of electrodes spaced apart in a square configuration, thereby increasing the required size of each micro-mirror. Moreover, the comb fingers for the tilt electrode extend perpendicular to the tilt axis and require relatively fine finger spacing, e.g. 3 um. Piano-MEMS micro-mirrors, which tilt about two perpendicular axes and can be tightly packed together, are disclosed in U.S. Pat. No. 6,934,439 issued Aug. 23, 2005 in the name of the present Applicant. A hidden hinge version of the piano-MEMS micro-mirrors is disclosed in United States Patent Publication 2007/0236775 published Oct. 11, 2007 in the name of the present Applicant. The aforementioned piano-MEMS devices pivot about a single centrally located post with the use of torsional hinges and a gimbal ring. Since these devices attract the lower surface of the mirrored platform toward the hot electrodes on the substrate, the precision and maximum tilt angle is limited by the size of the mirror.
Future MEMS mirror arrays for wavelength selective switching call for relatively long and stiff (thick) mirrors capable of tilting in two axes, and a relatively high tilt angle. Hidden hinge designs, in which the mirror is fabricated in a layer above the hinge plane, are attractive to reduce mirror mass moment of inertia and chip size, as the mirror need only be about the same size as the optically active area required.
Vertical comb drives provide relatively large electrostatic torque that is required for high tilt angle. Hidden hinges with vertical comb actuators are quite a powerful combination for next generation wavelength selective switches. It is also highly desirable to reduce complexity of vertical comb fabrication process.
An object of the present invention is to overcome the shortcomings of the prior art by providing a biaxially pivoting MEMS micro-mirror device in which the electrode surfaces are separate from the mirrored platform providing a large tilt angle, which is easily controllable and highly accurate. Another object of the present invention is to provide relatively long and thin micro-mirrors, which can be positioned very close together with only an air gap therebetween, by disposing both the roll and tilt electrodes along the same axis.